Apparatus and method for electroplating a wafer surface

ABSTRACT

The present invention provides for a method of electroplating a wafer surface comprising introducing a plating liquid to the wafer surface in a chamber and rotating the wafer about an axis of rotation passing through the surface to be plated, and including the further step of moving the wafer such that its axis of rotation is itself caused to rotate about a second axis of rotation.

The present invention relates to an apparatus and method for electroplating a wafer surface and comprising the step of introducing a plating liquid to the wafer surface in a chamber, and rotating the wafer about an axis of rotation passing through the wafer surface to be plated.

For many years now, there has been an increasing demand to reduce electronic component dimensions and thus the dimensions of products containing such components. This has lead to a continued desire to seek miniaturizing of integrated circuit devices. A particularly fruitful route that has been pursued in seeking to achieve miniaturization of electrical components is through the adoption and adaptation of lithographic and plating techniques commonly employed during electronic device production. Such plating techniques are generally employed to introduce a layer, and in some instances a patterned layer, of conductive material. Through appropriate lithographic patterning, and subsequent processing, a large number of component products can be manufactured by way of one or more common steps on, for example, a ceramic, glass or silicon wafer.

Insofar as a large number of devices are therefore formed on such a wafer, it is of course desirable to produce devices exhibiting at least very similar structural dimensions and closely similar operational characteristics.

However, somewhat disadvantageously, it has been found that, with known electroplating processes, the thickness profile of the deposited metal is disadvantageously non-uniform.

Such limitations in forming the conductive metal layer through electroplating therefore disadvantageously limit the number of useful functional devices that can be produced through such mass production methods and so adversely affect yield figures.

The amount of variation exhibited depends upon the particular electroplating bath used and also the manner in which the wafer is processed. Other factors include the size and shape of the electroplating cell, depletion effects, and so called hot-edge and terminal effects. The variations that do occur in electroplated layer thickness are generally expressed in terms of standard deviation and it is not unusual for known production methods to produce wafers exhibiting standard deviations that exceed 10%.

Current electroplating systems are known to employ a cell in which the plating fluid is caused to flow upwardly from a bottom region to a top region thereof. The wafer is located at the top of the cell and caused to rotate about an axis of rotation passing perpendicularly through the surface to be plated. As the plating fluid flows onto the wafer, the flow is deflected to the edges of the wafer and then exits the cell. However, through non-uniformities in the fluid-flow over the wafer, the problems of non-uniformity of electroplating result. The centre region of the wafer generally experiences a different flow-field from that of the peripheral regions of the wafer. More importantly, in such known arrangements, the positioning of the wafer is critical and even the smallest errors in such positioning can lead to large variations in electroplated layer thickness and thus undesired high values of standard deviation.

Such current systems are known from U.S. Pat. No. 6,017,437 and JP-A-04 041698.

More advanced systems have also been proposed suggesting the use of wafer handling techniques within the plating cell along with continuous plating bath analysis. Such aspects serve to monitor the progress of the electroplating in an attempt to make adjustments to the process in real time so as to increase uniformity and layer thickness and thus achieve low values of standard deviations. While it has been suggested that standard deviation values of 3% can be achieved, such systems are considered to be disadvantageously expensive and complex.

The present invention seeks to provide for an electroplating apparatus and method exhibiting advantages over known such apparatus and methods.

In accordance with one aspect of the present invention, there is provided a method of electroplating a wafer surface as defined above, and characterized by the step of further moving the wafer such that its axis of rotation is itself caused to rotate about a second axis of rotation.

In this manner, it is determined that the plating fluid can be caused to flow along the wafer surface in a highly turbulent manner and the two rotations defined serve to average out all non-symmetrical plating phenomena over the wafer. A uniform electroplating layer can then be developed.

Further, it has been found that the proposed system is relatively inexpensive and offers a relatively simple operational solution to the known problems and limitations. In particular, no continuous monitoring of the plating solution is required and the method can be employed either inside, or outside, a clean room environment. The method can be achieved by means of apparatus exhibiting a small footprint and is particularly suited for high volume production.

The feature of claim 2 is particularly advantageous in achieving uniformity of layer thickness.

The features of claims 3, 4 and 5 relate to particular advantageous embodiments of the present invention in achieving a compact, but effective, construction and method of operation.

According to another aspect of the present invention there is provided an apparatus for electroplating a wafer surface, including a chamber for housing a wafer and into which plating liquid is to be introduced, driven support means for supporting the wafer and rotating the same about an axis passing through the wafer surface to be plated, and characterized by further driven support means arranged to move the wafer support such that its axis of rotation itself rotates about a second axis of rotation within the chamber.

According to a yet further aspect of the present invention, there is provided a support apparatus for supporting a wafer having a surface to be plated, comprising first means arranged for supporting the wafer and for the rotation of the wafer about an axis of rotation passing through the wafer surface to be plated, and characterized by second means arranged for supporting the said first means and for moving the said first means in a manner such that the said axis of rotation of the first means rotates about a second axis of rotation.

According to another aspect of the invention, there is provided a method of manufacturing an electronic device including the step of electroplating a wafer surface. This leads to layers with a more uniform thickness. Thus yield can be optimized and alignment can be improved. Further on, during wet etching, the size pattern is dependent on the layer thickness. More uniform thickness thus lead to more uniform and better controllable pattern sizes. Examples of electronic devices are semiconductor devices, passive networks, microelectromechanical components and the like.

The invention is described further hereinafter, by way of example only, with reference to the accompanying drawings, in which:

FIG. 1 is a diagrammatic cross-sectional view through a cell forming part of a plating apparatus embodying the present invention;

FIG. 2 is a plan view of the wafer support and supporting plate employed within the embodiment of FIG. 1;

FIG. 3 is a graphical representation of the variation in cross-sectional thickness across a wafer plated in accordance with a method embodying the present invention and illustrating the values of table 1; and

FIG. 4 is a graphical representation of all of the thickness data for the wafer plating of FIG. 3.

Turning first to FIG. 1, there is illustrated in cross-section a view of an electroplating cell 10 forming part of a system embodying the present invention.

The electroplating cell comprises a bath 12 having an inlet duct 14 extending upwardly in a vertical manner from a central portion of the lower surface of the bath 12. The inlet duct 14 allows for the introduction of plating liquid into the bath 12 in a manner commonly known in the art. The plating liquid is introduced so as to flow into, and throughout, the bath 12 in a manner indicated by arrows A. As can be seen, the plating liquid therefore circulates either side of the inlet duct 14 and flows out of the bath 12 by way of an outlet aperture 16 located in the upper peripheral region of the bath 12. The outlet aperture 16 is also defined by a laterally extending wafer-mounting arrangement which, in combination with the bath 12, serves to define the dimensions of the electroplating cell 10. The arrows A also serve to indicate how the plating fluid flows in the bath in the region immediately adjacent the wafer-mounting arrangement 18. In further detail, and as an important aspect of the present invention, the mounting arrangement 18 includes a circular mounting plate 20 which depends from, and drivingly engages, a shaft 22 which is caused to rotate in the direction of arrow B by means of a motor arrangement (not shown).

Rotatably mounted within the mounting plate 20 is a circular wafer-holder 24 having a lower surface 26 which, in combination with a lower surface of the mounting-plate 20, defines the upper surface of the bath 12. The circular wafer-holder 24 is connected to a shaft 16 which is in turn connected to a driving arrangement such as a motor (not shown) which serves to rotate the wafer-holder 24 in the direction of arrow C.

In the illustrated example, the wafer-holder 24 forms the cathode and a lower surface of the bath 12 is arranged to include a laterally extending anode 28.

The relative locations, and rotations, of the mounting member 20 and the circular wafer holder 24 are illustrated further with reference to FIG. 2.

As will be appreciated, while the driven shaft 16 serves to rotate the wafer holder 24 in a manner indicated by Arrow C, the driven shaft 22 likewise serves to rotate the mounting plate 20 in the opposite direction of arrow B. This effectively causes the rotating circular wafer mounting plate 24 to orbit around the central shaft 22 following a path along the peripheral region of the cell 10.

In effect, the axis of rotation of the wafer mounting plate 24 is itself caused to rotate around the axis of rotation of the mounting plate 20, in the illustrated example, the two said axes of rotation being substantially parallel. Thus, the wafer (not shown) mounted on the underside 26 of the wafer holder 24 thereby moves around the upper region of the cell 10 in a manner so as to create a flow of the plating fluid over the surface of the wafer. Turbulence can be introduced into the fluid by means of the pump for introducing the fluid into the cell.

It is found that mounting and rotating the wafer in this manner relative to the bath imparts advantages to the present invention when compared with the prior art. In particular, the manner in which the wafer is thus rotated serves to average-out all generally non-symmetrical phenomena that might otherwise arise during the electroplating process.

The advantages of the present invention can therefore be achieved by means of a relatively simple, and cost-effective, apparatus and employing the standard plating solution. Continuous monitoring of the plating solution is not required and the apparatus can advantageously be used outside of a clean-room environment. The apparatus can advantageously exhibit a small footprint and so can be particularly suited for use for high volume production requirements.

As should be appreciated, the anode 28 has a large surface area and is preferably formed of an inert material such as gold. Flow measurements along the wafer indicate a well-developed turbulent flow and it has been found that the system can advantageously be operated in two modes. In a first, i.e. DC mode, a constant current is imposed between the anode 28 and cathode 24. Alternatively, in a Reverse Pulse Plating (RPP) mode, the current is periodically reserved between the anode 28 and cathode 24. It is noted that the intensity of the anodic current is different from the cathodic current and also the anodic and cathodic times are found not to be equal. Tests have been conducted on a system embodying the present invention and in respect of two different wafers.

First, a wafer without structures formed thereon was plated and the lowest standard deviation in the RPP mode was found to be 2.5%. Such results are further illustrated in table 1 below and also with regard to FIGS. 3 and 4 which indicate the layer thickness distribution of a 7.5 um copper deposition layer in RPP mode on a 6 inch (15.24 cm) wafer. TABLE 1 Average Stdev Rel Stdev Min Max Left-Right 7.44 0.14 1.83 7.19 7.82 Top-Bottom 7.46 0.20 2.66 7.19 8.14 Total 7.47 0.17 2.24 7.17 8.14

Also, the wafers that were plated and which had lithographically formed microstructure thereon, also exhibit a standard deviation in the DC mode in the range of 2.5%. The layers were plated with a layer of copper. However the materials such as chrome nickel and alloys, may be plate as will with the method of the invention.

These low figures of standard deviation serve to confirm the advantageous nature of the present invention.

In short, the invention relates to an electroplating apparatus and a method for electroplating with this apparatus. The apparatus is provided with first driven support means to rotate a wafer (on a support plate). It is further provided with second driven support means to rotate the wafer about a second axis of rotation within the chamber.

Thus:

-   -   it is suitable for the electroplating of wafer, of any size,         particularly from 4 to 14 inch.     -   the wafer is present on a rotatable plate, which rotatable plate         is connected to a support plate that is also rotated. The wafer         is thus subject to two rotary movements, that can be different         in speed, in direction and with respect to the location of the         rotation axis.     -   the complete system of the support plate and the rotatable plate         is present in one chamber.

In order to discuss the validity, we will first discuss the novelty, and thereafter inventive step. Novelty is assessed by checking whether the invention is known completely from one document. Inventive step is assessed by checking whether a person skilled in the art could have derived without any problem the invention by combining the available documents. 

1. A method of electroplating a wafer surface comprising introducing a plating liquid to the wafer surface in a chamber and rotating the wafer about an axis of rotation passing through the wafer surface to be plated, characterized by the step of further moving the wafer such that its axis of rotation is itself caused to rotate about a second axis of rotation.
 2. A method as claimed in claim 1, wherein the wafer is located between the said axes of rotation and outer walls of the chamber.
 3. A method as as claimed in claim 1, wherein the wafer is located on a substantially circular wafer support and the wafer's axis of rotation passes through the centre of the circular support.
 4. A method as claimed in claim 3, wherein the said substantially circular wafer support holding the wafer is itself arranged to be supported on a circular support plate arranged to rotate about the said second axis of rotation.
 5. A method as claimed in claim 4, wherein the said substantially circular wafer support, the wafer and the said circular support plate are driven to rotate as rotating eccentric circles.
 6. A method as claimed in claim 1, wherein the respective rotations about the said axes of rotation are in opposite directions.
 7. A method as claimed in claim 1, wherein the said second axis of rotation is substantially parallel to the said axis of rotation passing through the wafer.
 8. An apparatus for electroplating a wafer surface, including a chamber for housing a wafer and into which plating liquid is to be introduced, driven support means for supporting the wafer and rotating the same about an axis passing through the wafer surface to be plated, and characterized by further driven support means arranged to move the wafer support such that its axis of rotation itself rotates about a second axis of rotation within the chamber.
 9. An apparatus as claimed in claim 8, including means for locating the wafer between the said second axes of rotation and an outer region of the chamber.
 10. An apparatus as as claimed in claim 8, wherein the driven support means for supporting the wafer comprises a circular member.
 11. An apparatus as claimed in claim 10, wherein the said further driven support means comprises a circular member.
 12. An apparatus as claimed in claim 11, wherein the two circular members are arranged as eccentric circular members.
 13. An apparatus as claimed in claim 8, wherein the respective rotations around the axes of rotation are arranged to be in opposite directions.
 14. An apparatus as claimed in claim 8, wherein the said second axis of rotation is substantially parallel to the said axis of rotation passing through the wafer.
 15. Support apparatus for supporting a wafer having a surface to be plated, comprising first means arranged for supporting the wafer and for the rotation of the wafer about an axis of rotation passing through the wafer surface to be plated, and characterized by second means arranged for supporting the said first means and for moving the said first means in a manner such that the said axis of rotation of the first means rotates about a second axis of rotation.
 16. An apparatus for electroplating a wafer surface, including a chamber for housing a wafer and into which plating liquid is to be introduced, driven support means for supporting the wafer and rotating the same about an axis passing through the wafer surface to be plated, and characterized by further driven support means arranged to move the wafer support such that its axis of rotation itself rotates about a second axis of rotation within the chamber, wherein the respective rotations around the axes of rotation are arranged to be in opposite directions, the apparatus configured to implement a method of electroplating a wafer surface comprising introducing a plating liquid to the wafer surface in a chamber and rotating the wafer about an axis of rotation passing through the wafer surface to be plated, characterized by the step of further moving the wafer such that its axis of rotation is itself caused to rotate about a second axis of rotation, wherein the said second axis of rotation is substantially parallel to the said axis of rotation passing through the wafer.
 17. Support apparatus as claimed in claim 15, wherein the said second axis is substantially parallel to the axis of rotation of the said first means.
 18. A method of manufacturing an electronic device comprising the step of electroplating a wafer surface according to claim
 1. 19. A method of manufacturing an electronic device comprising the step of electroplating a wafer surface according to claim
 2. 20. A method of manufacturing an electronic device comprising the step of electroplating a wafer surface according to claim
 3. 